This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 Appln. No. 00111617.7 filed in Europe on May 31, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to a method of adjusting sizes of cooling holes of a gas turbine component, and more particularly, a method of adjusting sizes of cooling holes in a component having at least one protective coating on an outer surface and/or an inner surface, the component being used in a high temperature environment.
Components such as gas turbine blades, vanes and other cooled parts often contain cavities that distribute cooling air to a plurality of holes in the wall of the part that lead to the outer surface. Most turbine components are coated for protection from oxidation and/or corrosion with, for example, a MCrAlY coating (base coat) and some are also coated with a thermal barrier coating (TBC) for thermal insulation.
Cooling holes are sometimes manufactured larger than the intended size limit. This could happen with a new component during the production of the holes. The problem is that enlarged cooling holes lead to an increased flow of cooling air and an altered film cooling air distribution, which can cause a changed cooling efficiency of the components at different locations. Thus, the overall efficiency can be affected in an unintended and negative manner as a result of using an excessive amount of cooling air. The enlarged cooling holes can also result in increased degradation of the base component at locations where there is a reduced cooling effect. Therefore, the present invention provides an easy method to restore the size of the cooling holes in new components to the originally intended limits and avoids a high rate of scrapping during production of the new parts, particularly during production start-up when errors occur often.
Furthermore, the same problems as discussed above can occur with components in which the size of cooling holes has been changed during service. This can happen due to oxidation, spallation and degradation of the base material exposed on the cooling hole surfaces. The present invention provides a method to repair these components rather than to use new parts by restoring the size of the cooling holes to a certain limit. Although there are several methods known in the art for repairing the protective coating itself, the known methods avoid the coating of the cooling holes. The known methods, such as disclosed e.g. in U.S. Pat. Nos. 4,743,462 and 5,800,695 do not provide methods for adjusting the size of cooling holes to bring them back to a given tolerance.
The present invention provides a method of adjusting the size of cooling holes of a gas turbine component to a size within manufacturing tolerances. The method can be applied with cooling holes which are initially manufactured with a greater size than desired, or with cooling holes which have an increased diameter due to oxidation, spallation or other factors, and must be serviced.
In the present invention a method is provided for restoring cooling holes in a component to within a desired tolerance. The component is shielded on at least the inside cavity or cooling passage into which the cooling holes are connected with a coating of wax or plastic or other easily removable and non-conducting material. The component is then subjected to a coating process, such as galvanic coating or a vapor phase coating process, in which a metallic coating is applied to the inside of the cooling holes to a thickness that restores the cooling holes to intended design manufacturing tolerances, after which the shielding material is removed. A heat treatment may be applied to the component to ensure the quality of the bonding of the newly applied metallic coating to the component.
The method according to the present invention has an advantage of bringing the size of the cooling holes to a desired tolerance relatively easily. This applies for both cooling holes manufactured with a size greater than intended originally or for cooling holes which have a greater size due to oxidation or spallation during service. In the last case it might be necessary to remove remaining oxidized material before applying the process by any suitable means such as acid cleaning, grit blasting or fluoride ion cleaning.
Where the component has two protective coatings, a metallic and a ceramic at the outside, the component is shielded during the method according to the invention with the shielding material provided only at the inside. On the other hand, where the component has only one protective coating on the outside, and does not include a ceramic coating, the component is shielded with the shielding material provided at both the inside and the outside.
In one embodiment of the invention, the metallic coating could have the following composition (wt-%): 25% Cr, 5.5% Al, 2.7% Si, 1.0% Ta, 0.45-0.8% Y, max. 0.03% C, balance Ni and unavoidable impurities. Also possible is a metallic coating such as Pt or Ni-based and containing at least one of the following components Cr, Al, Y. When the metallic coating is Pt, the Pt is applied by means of a galvanic process, and this could be followed by an aluminizing process using pack aluminizing, above-the-pack aluminizing or any other means for depositing on top of the already deposited Pt.
In another possible embodiment the metallic coating is applied by means of a high temperature chemical vapor deposition or any other gas phase means, and the portions of the component not desired to be coated are protected with a high temperature resistant masking material.
In a preferred embodiment the base component could have one of the following compositions:
A. max. 400 ppm C, 65 ppm B, (wt.-%) 0.2% Hf, 6.4% Cr, 9.6% Co, 0.6% Mo, 6.4% W, 6.5% Ta, 2.9% Re, 5.6% Al, 1.0% Ti,
B. 700 ppm C, 150 ppm B, (wt.-%) 1.4% Hf, 8.0% Cr, 9.0% Co, 0.5% Mo, 10.0% W, 3.2% Ta, 5.6% Al, 0.7% Ti,
C. max. 1600 ppm C, 150 ppm B, (wt.-%) 1.4% Hf, 8.4% Cr, 10.0% Co, 0.7% Mo, 10.0% W, 3.0% Ta, 5.5% Al, 1.0% Ti